Modern petroleum drilling and production operations demand a great quantity of information relating to parameters and conditions downhole. Such information typically includes characteristics of the earth formations traversed by the wellbore, along with data relating to the size and configuration of the borehole itself. The collection of information relating to conditions downhole, which commonly is referred to as “logging”, can be performed by several methods.
In conventional oil well wireline logging, a probe or “sonde” housing formation sensors is lowered into the borehole after some or all of the well has been drilled, and is used to determine certain characteristics of the formations traversed by the borehole. The upper end of the sonde is attached to a conductive wireline that suspends the sonde in the borehole. Power is transmitted to the sensors and instrumentation in the sonde through the conductive wireline. Similarly, the instrumentation in the sonde communicates information to the surface by electrical signals transmitted through the wireline. Since the sonde is in direct electrical contact with the surface installation, the communications delay is negligible. Accordingly, measurements can be made and communicated in “real time”.
A disadvantage of obtaining downhole measurements via wireline is that the drilling assembly must be removed or “tripped” from the drilled borehole before the desired borehole information can be obtained. This can be both time-consuming and extremely costly, especially in situations where a substantial portion of the well has been drilled. In this situation, thousands of feet of tubing may need to be removed and (if offshore) stacked on the platform. Typically, drilling rigs are rented by the day at a substantial cost. Consequently, the cost of drilling a well is directly proportional to the time required to complete the drilling process. Removing thousands of feet of tubing to insert a wireline logging tool can be an expensive proposition.
As a result, there has been an increased emphasis on the collection of data during the drilling process. Collecting and processing data during the drilling process eliminates the necessity of removing or tripping the drilling assembly to insert a wireline logging tool. It consequently allows the driller to make accurate modifications or corrections as needed to optimize performance while minimizing down time. Designs for measuring conditions downhole including the movement and location of the drilling assembly contemporaneously with the drilling of the well have come to be known as “measurement-while-drilling” techniques, or “MWD”. Similar techniques, concentrating more on the measurement of formation parameters, commonly have been referred to as “logging while drilling” techniques, or “LWD”. While distinctions between MWD and LWD may exist, the terms MWD and LWD often are used interchangeably. For the purposes of this disclosure, the term LWD will be used with the understanding that this term encompasses both the collection of formation parameters and the collection of information relating to the movement and position of the drilling assembly.
A number of measurement techniques have been used in wireline and/or LWD measurements. These include, among others, induction, resistivity, permittivity, magnetic permeability, acoustic speed, nuclear magnetic resonance (NMR), gamma radiation (GR) and thermal neutron delay (TMD) measurement techniques. One issue that logging tools making these measurements must address is relative dip. The borehole typically intersects formation beds at an angle other than the ideal ninety degrees. Consequently, formation boundaries intersect the borehole at an angle, making the transition between formation layers appear more gradual than is actually the case. Techniques exist for measuring formation dip angles and using the dip angle measurements to compensate for dip angle effects on the logging tool measurements.
Another problem with the logging tools that make these measurements is a limited vertical resolution. Each tool has a “tool response” that extends over a measurement region, causing the tool to provide a measurement that represents a weighted average of material properties in that region. This averaging effect “smears” the property measurements, and creates the possibility that fine-resolution features (e.g., thin beds and sharp boundaries) may be missed.
Removal of the smearing effect is known as deconvolution or inversion. Deconvolution is known to be “an ill-posed problem” in that any number of formations could have produced the measured log. The presence of measurement nonlinearities further complicates the deconvolution problem.
A number of inversion techniques exist for removing the smearing effect of the tool response. Several are described in U.S. Pat. No. 5,867,806, entitled “System and Method for Performing Inversion on LWD Resistivity Logs with Enhanced Resolution,” by R. W. Strickland, et al, which is hereby incorporated by reference.
“Forward Deconvolution” is a preferred technique for enhancing tool resolution. In the forward deconvolution technique, an approximate or estimated model of the formation is made from the data logs. This model of the formation is essentially an estimate of the characteristics of the formation. After the model of the formation is generated, a computer model of the tool response is used to transform the estimated model of the formation into an estimated log. This estimated log is then compared with the actual log data. One or more parameters of the formation model are then adjusted based on this comparison of the estimated log to the actual log data, a new estimated log is calculated, a new comparison is made, and the process repeats. Thus, the forward deconvolution technique iteratively refines the formation model until the simulated log approximates the actual log. This technique can be computationally intensive, but may offer some advantages over other inversion approaches. Among these potential advantages are flexibility and stability in the presence of nonlinearities.
The existing inversion techniques have been found to yield inadequate resolution, in that thin beds are routinely overlooked. These thin beds may contain retrievable oil or other hydrocarbons. An inversion technique that offers improved resolution without instability would be desirable.